This invention relates to flame photometers in automatic chemical analyzers and more particularly to improved ignition electrode assemblies therein.
Flame photometers of the type contemplated are primarily utilized for measuring sodium and potassium levels in human serum. A gas flame is provided, such as from a propane source, and ignited at a screen at an end of a gas nozzle. The screen is important in that it shapes the flame so that appropriate wavelength filters and photocells may be positioned with respect to the flame for reading the intensity at a selected wavelength of radiation emitted by samples introduced into the flame. Normally a flame diluent is provided and vapors thereof are put into an air path passing along the flame, and colors produced thereby are read by the photocells. A suitable flame diluent is a solution of lithium carbonate in water. Lithium produces a good background wavelength, i.e. it will screen out undesired background light and does not produce a wavelength in the same area of interest as sodium and potassium. Lithium is also a good selection since lithium will not be found in human serum except perhaps in the cases of persons taking certain psychedelic drugs.
Conventionally, igniter means are provided adjacent an upper screen for igniting the gas at the initiation of operation. Igniter means may commonly comprise, for example, an electrode mounted on or adjacent the screen and a second electrode mounted at a point remote therefrom. An important constraint on mounting a remote electrode is that it must be mounted outside of the path of the flame so as not to interfere therewith but must not be mounted too far from the screen. Mounting an electrode too far from the screen requires a very large voltage to break down and ionize gas in order to produce a spark. Also, some electrode arrangements which require a relatively large amount of gas flow before ignition present the potential for a flare-up upon the ignition. Another desirable characteristic is that the electrodes be of the type whose operation is not impeded under adverse conditions of flame burning. More specifically, if a flame burns too leanly, various components chemicals in the vapor passing over through the flame will deposit on the electrode. Where the electrode consists of an exposed point rather than an exposed larger surface, the electrode may be covered with deposits and require either greater voltage for operation or perhaps even cease operation. Another desirable characteristic is that the electrodes be easily assembled. Further, should an electrode require a replacement it is desirable to have a system in which minimum disruption to the flame photometer system is required.